Manufacture of paper cups begins with rolls of paperboard stock. Generally, the stock is first coated with polyethylene on at least the surfaces that will be the inside cup surfaces (in the case of a cup for hot liquids) and on both inside and outside surfaces of cups intended for cold liquids. The outside of cold cups needs to be coated because condensation that forms on the outside of a cup holding a cold beverage or other liquid can soak into the paperboard of a cup not coated on the outside. The polyethylene coated stock is then printed with any printing to appear on the finished cup.
After coating and printing, the printed paperboard stock is die cut into flats that will become the cup wall. Each flat is then wound around a tapered mandrel to form the cup wall, and overlapping wall edges are bonded with heat and pressure.
Disks for bottoms are die cut from typically unprinted paperboard stock, and a disk is pressed into the smaller diameter of each cup wall and sealed in place with heat and pressure. Finally, the upper edge of the cup is rolled into a lip.
The bonds between overlapping edges of the cup wall are formed between a polyethylene coated surface and an uncoated surface in the case of a typical hot cup and between two coated surfaces in the case of a cold cup. At least a portion of the bond between the cup bottom and the cup wall are formed between surfaces coated with polyethylene in all paper cups, and the entire bottom bond in a cold cup is between two coated surfaces. These bonds are typically adequate to avoid leaks in smaller, shorter cups.
However, the liquid pressure at the bottom of a tall cup becomes significant when full, with the result that leaks sometimes develop where the cup bottom is bonded to the sidewall. Leaks may also develop where difficult shapes have been bonded.